Abstract text: Explosive seed dispersal distinguishes Cardamine species from Arabidopsis. This trait depends on a polar secondary cell wall (SCW) in endocarp b (endb) cells, yet its environmental and genetic basis remains unclear. Cardamine chenopodiifolia is a polyploid that produces explosive fruit above ground and non-explosive fruit below ground. We show light triggers underground fruit to explode by switching SCW patterning from uniform to polar in endb cells. We identify the HD-ZIPIII gene REVOLUTA as a central regulator of endb cell fate, SCW formation, and organ polarity in Arabidopsis and Cardamine hirsuta. In C. hirsuta, duplicated REVOLUTA paralogs are required for SCWs, whereas other HD-ZIPIII genes contribute redundantly to cell fate and organ polarity. Loss of both REVOLUTA paralogs eliminates endb SCWs, whereas over-expression converts polar endb SCWs to uniform, producing non-explosive fruit. Tuning REVOLUTA activity modulates seed dispersal strategy and suggests a genetic route for trait transitions during polyploid evolution.